1. Field
Medical/dental implants.
2. Background
Metal implants are widely used in medical and dental applications, such as in orthopedic hip and knee surgeries and in dental surgery. Over two million orthopedic procedures and over 10 million dental implant procedures are performed in the United States every year. Implants fail because of poor osseointegration between the implant and the natural bone. Implants are typically made of metal materials, with titanium (Ti) and its alloys being favored due to their biocompatibility and mechanical properties. For the implants to function successfully, a direct chemical bond between the implant and the bone needs to form rapidly and needs to be retained over many years while the implant is loaded. Metal materials, however, do not form a direct chemical bond with bone. In order to promote osseointegration between the metal implant and bone, a layer of osseointegration promotion material is incorporated on the implant. Calcium phosphate ceramic materials are an example of coating materials that promote osseointegration. The most popular coating among the calcium phosphate family is hydroxyapatite (HA) due to its chemical stability and osteoconductivity.
Important parameters in the long-term behavior of implants coated with HA include at least acceptable coating-substrate bond strength and biostability (i.e., a low dissolution rate of the coating). In order to improve coating-substrate (usually a metal) bond strength and other properties, a variety of coating techniques have been explored to develop thin (generally less than 10 microns) coatings of HA and other calcium phosphates. U.S. Pat. No. 4,908,030 discloses a method of forming a thin HA coating on an implant using ion beam sputtering. U.S. Pat. No. 5,817,326 discloses a method in which one or more layers of HA sol-gel are cured to densify on a titanium alloy implant, followed by a non-line-of-sight ion implantation process, in order to strengthen the adhesion of the HA coating to the substrate. U.S. Pat. No. 5,543,019 discloses a method of forming a thin coating layer on the surface of an implant using a plasma sputtering process. Other methods developed include pulsed laser deposition and magnetron sputtering.
Another approach to improve the bonding capability of an HA coating onto a metallic substrate has been the deposition of a composite coating, wherein a metallic phase is introduced to serve as either an intermediate layer or a second (continuous or dispersed) phase in an HA matrix. For example, Dasarathy et al., in “Hydroxyapatite/metal composite coatings formed by electrocodeposition,” J. Biomed. Mater. Res., 31, 81–89 (1996), describes an electro-codeposition process to coat a cobalt/HA (Co/HA) composite coating on a Ti substrate with a bond strength up to 37 MPa. Using plasma spray technique, Brossa et al., in “Adhesion properties of plasma sprayed hydroxyapatite coatings for orthopaedic prostheses,” Bio-Med. Mater. Eng., 3, 127–136 (1993), and Nakashima et al., in “Hydroxyapatite coating on titanium-sprayed titanium implant,” in Bioceramics 6, P. Ducheyne and D. Christiansen (eds.), Butterworth-Heinemann, Oxford, 1993, pp. 449–453, describes a double-layer coating including an HA layer on top of a porous Ti precoat on a Ti substrate. This double-layered coating was shown to outperform a monolithic HA coating in adhesion properties. German patent “Coating of implants,” Gruner, Heiko (Plasmainevent A.-G.) Ger. Offen. DE 3,516,411 (C1. C23C4/04) Nov. 12, 1986, describes a multi-layered coating comprising a Ti precoat, a Ti/HA composite layer and an HA overlayer formed by plasma deposition. The multi-layer coated implants show fast and stable fusion between the coated implant and the bone. On Ti—6A1—4V substrate Ferraris et al., in “Vacuum plasma spray deposition of titanium particle/glass-ceramic matrix biocomposites,” J. Am. Ceram. Soc., 79, 1515–1520 (1996), plasma-sprayed a Ti particle-reinforced bioactive glass composite coating, which exhibited a higher bond strength than that of monolithic bioactive glass coating.
Pure titanium implants have become a preferred choice instead of calcium phosphate coated implants in recent years because of the critical disadvantages of previous calcium phosphate coatings. Plasma spraying and sputter coating are two major techniques that were widely used for HA coatings. These methods tend to have a problem with the dissolution of calcium phosphate, at a 50 percent rate, through a high temperature processing. Different phases of non-HA phosphate calcium sprayed on implants are easy to dissolute in body solution. The calcium phosphate coated implant by these methods also failed in long term stability often due to fracture at the coating-titanium interface which appeared to have been caused by the poor adherence of the HA film to the metal substrate. Furthermore, sputter plasma and coating tend to produce a non-uniform coating when they are applied to an irregular or porous surface.
Accordingly, a need exists for an implant with an improved bond strength and biostability as well as a method of forming such implants for use in orthopedic and dental applications. In addition, a majority of commercially available titanium implant systems utilize some degree of macroporous surface topography based on preclinical and clinical data that a roughened surface topography appears to enhance the rate and speed of functional implant-tissue integration. A conformer coating on the rough implant surface is needed.